Environmental Technology: Development of an Alum Sludge-Based Constructed Wetland System for Improving Organic Matter and Nutrients Removal in High-Strength Wastewater

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The generation of alum sludge in Ireland is inevitable. Its disposal, however amidst decreasing available landfill space coupled with escalating costs and public concerns remains an environmental quagmire. Seeking costeffective and environmentally sustainable disposal alternatives is, as a result, a significant environmental issue. Alum sludge is a by-product of drinking-watertreatment processes in Ireland. It is obtained when aluminium (Al) salts are used as a chemical coagulant. Once alum sludge is dewatered, however, aluminium hydroxides become the dominant constituent, making it possible to reuse the sludge as a valuable raw material in wastewater treatment. This is because the ions enhance adsorption and chemical precipitation processes that remove various pollutants, especially phosphorus, from wastewater. Moreover, there is huge potential in Ireland for the application of small wastewater treatment systems, especially constructed wetlands (CWs), because of the unique geographic distribution of Irish residents. Constructed wetlands (commonly known in Europe as ?treatment reed beds?) are regarded as a low-cost promising technology for wastewater treatment and have been increasingly employed for wastewater treatment since the early 1990s. This project conceptualised and developed a novel CW system integrating alum sludge as its main substrate. The system has a multiprong feature of creating an alternative reuse option for alum sludge (as opposed to landfilling it) and, at the same time, enhancing wastewater treatment with high rates of phosphorus (P) removal. The system can be applied successfully to treat wastewater in various situations in Ireland, including individual houses, farms and scattered settlements. Furthermore, the concept of reusing alum sludge for P immobilisation introduced in this study can be integrated into any form of CW in Ireland to enhance P removal. The results obtained both in the laboratory and on the field have shown that the alum sludge-based CW system is a promising new technology that is green, low cost and environmentally friendly. Moreover, the results also confirm that the system can concurrently enhance the removal of P and organic matter from wastewater with very high removal efficiencies. Data obtained from the laboratory-scale trials indicate that average removal efficiencies for 5-day biochemical oxygen demand (BOD5) (90.6%), chemical oxygen demand (COD) (71.8%), reactive phosphorus (RP) (93.3%) and soluble reactive phosphorus (SRP) (97.6%) were achieved. A similar treatment performance was obtained from the field trials of the system after one year of operation. At a daily wastewater load of 990 L, a range of mean monthly removal efficiencies of 57% to 84%, 36% to 84% and 73% to 97% were achieved for BOD5, COD and SRP respectively.

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Zhao, Y. Babatunde, A.

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Environmental Technology: Development of an Alum Sludge-Based Constructed Wetland System for Improving Organic Matter and Nutrients Removal in High-Strength Wastewater

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Description of Geographical Characteristics of This Project or Dataset Field trials of the pilot-scale systems derived from the laboratory scale development were conducted on an animal farm as a demonstration study for further development. The potential for wetland clogging and the possible release of substances/elements (especially Al) from the alum sludge into the treated wastewater upon passage through the system was also examined. According to an inventory survey carried out in 2005 at University College Dublin (UCD), there are now over 140 CW sites in Ireland (Babatunde et al., 2008a). The two counties account for about 30% of all the CWs in Ireland. Both Wexford and Cork are predominantly rural areas. However, while 47% of the systems in Wexford are used for treating wastewater emanating from dwellings, approximately the same numbers of systems (44%) are used for commercial purposes in Cork. In the urban areas, it would seem that conventional wastewater treatment technologies are still dominant, for example in Co. Dublin, which has only about 2% of the total number of CW. Therefore, given the economic constraints of installing conventional wastewater treatment systems in most cases, CW would be a costeffective land-use practice, which could help retain the nutrients and improve surface water quality. After extensive laboratory-scale investigations, a pilotscale field study was conducted on an animal farm with real wastewater for 10 months. The purpose of the field study was to: (i) validate the achievement obtained in the laboratory study, and (ii) gain experience from the engineering point of view especially for the possible large-scale application of the CW. The field-scale system was constructed on a farm in Newcastle, Co. Dublin, Ireland. It consisted of four treatment stages constructed using similar The alum sludge cakes used were collected fresh from the industrial filter press of the sludge dewatering unit of the Ballymore-Eustace Water Treatment Plant in south-west Dublin, Ireland where aluminium sulphate is used as coagulant 1100 L plastic bins.

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In Ireland alone, an estimated 15,000 to 18,000 tonnes of dried solids of alum sludge are produced annually, and this is expected to double by the end of the next decade. In most countries, including Ireland, alum sludge is currently treated as a waste and it has become mandatory for water companies to dispose of the dewatered sludge appropriately. Most sludge is disposed of into landfills. The increasing amounts of alum sludge produced daily are triggering considerable environmental and economic concerns in addition to disposal issues. However, the options for alum sludge recovery/recycling are generally not well developed as economically viable options for application in Ireland. Thus, the search for cost-effective and eco-friendly Globally, there is a need to reuse industrial by-products in line with moves towards sustainable development and environmental policies of ?reduce, reuse and recycle?. This research examined the possible reuse of the Irish dewatered alum sludge as a low-cost alternative substrate in CW, particularly for P removal.

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